Cycle life, capacity fade and warranty terms explained
Published on September 20, 2026
Every supplier claims "6000 cycles" and "5-year warranty". But what does that actually mean in years of service — and who pays when capacity fades? This guide explains cycle life, the fade curve, and how to read warranty terms before you buy.
1. What cycle life really means
A cycle is one full discharge to the defined depth and recharge back to full. If you only use half the capacity each day and recharge, two such days count as roughly one full cycle.
6000 cycles (typical for LiFePO4): at one full cycle per day that is theoretically about 16 years; at two cycles per day, about 8 years.
Cycle-life figures come from laboratory conditions (defined temperature, C-rate and depth of discharge). Real-world results are usually somewhat lower.
2. Cycle life is not the same as calendar life
Cycle life: determined by how many times you charge and discharge.
Calendar life: batteries age even when idle — typically 10–15 years for LiFePO4.
Whichever comes first governs. For C&I projects cycling daily, cycle life usually dominates; for backup systems that rarely discharge, calendar life is the limiting factor.
3. Capacity fade and "end of life"
The industry generally defines end of life (EOL) as remaining capacity falling to 70–80% of the original rating. Typical fade behaviour:
Slightly faster fade over the first few hundred cycles, then a long slow plateau.
Noticeably faster decline near end of life.
Most warranties promise "no less than 70% or 80% of initial capacity" at the end of the term.
4. Four factors that drive lifespan
Depth of discharge (DOD): deeper cycles wear the battery faster. Regular 100% deep discharge shortens cycle count; shallow cycling lasts longer.
Temperature: heat is the number one enemy. Sustained operation above 35°C accelerates fade. Cold mainly reduces available capacity, and charging at high current in cold conditions can cause damage.
C-rate: sustained high-current charge/discharge increases wear — stay within the design rating.
Cell consistency: poorly matched packs see individual cells age early and drag the whole pack down — the main reason to insist on Grade-A cells.
5. How to read a warranty
A clear warranty states three parallel conditions:
Time limit: for example 5 or 10 years.
Cycle limit: for example 6000 cycles.
Retained capacity: for example at least 70% of initial capacity.
Whichever is reached first applies. So a battery used heavily could exhaust its cycle allowance after only 3 years, even though the calendar term has not expired.
6. Common warranty traps
Years only, no capacity commitment: if capacity drops to 60%, the supplier can argue the battery "still works".
No cycle limit stated: high-frequency C&I projects can burn through cycles in a few years.
Hidden operating conditions: requirements on temperature range, certified installers or complete maintenance records, otherwise the claim is void.
Cells only, not the system: PCS, BMS and air-conditioning left out of coverage.
Vague remedy: full pack replacement, faulty module only, or depreciated compensation — this must be spelled out.
7. Buyer's checklist
Are all three numbers written down: years, cycles and retained capacity?
Does coverage include the whole system (battery + PCS + BMS + thermal management)?
Are the cells Grade-A with third-party test reports?
Are operating conditions clear (temperature range, C-rate)?
Is the remedy, response time and spare-parts policy defined?
Does the supplier have after-sales and technical support capacity — critical for overseas projects.
Need clear warranty terms and cell test reports? We provide written warranty commitments, cell test reports and a defined after-sales support plan.